Handling JSF session timeouts

Question

I'm having a problem when my session in jsf times out. I have a logoutBean that handles the logout functionality for my app. The problem comes in when the session expires after period of time with no activity on the app. If i try then call the logout() method in my logoutBean i get the following error in Tomcat5.5 listed below (this is only a short extract, the exception thrown is a lot longer) Any idea on how to check if the session has expired before you submit a request??
SEVERE: Managedbean TopBean could not be created Can't instantiate class: 'za.co
.vodacom.rms.frontend.bean.TopBean'.. class za.co.vodacom.rms.frontend.bean.TopB
ean : java.lang.NullPointerException
javax.faces.FacesException: Can't instantiate class: 'za.co.vodacom.rms.frontend
.bean.TopBean'.. class za.co.vodacom.rms.frontend.bean.TopBean : java.lang.NullP
ointerException
at com.sun.faces.config.ManagedBeanFactory.newInstance(ManagedBeanFactor
y.java:229)
at com.sun.faces.application.ApplicationAssociate.createAndMaybeStoreMan
agedBeans(ApplicationAssociate.java:306)
at com.sun.faces.el.VariableResolverImpl.resolveVariable(VariableResolve
rImpl.java:97)
at com.sun.faces.el.impl.NamedValue.evaluate(NamedValue.java:145)
at com.sun.faces.el.impl.ComplexValue.evaluate(ComplexValue.java:166)
at com.sun.faces.el.impl.ExpressionEvaluatorImpl.evaluate(ExpressionEval
uatorImpl.java:263)

Answer 1

Here's a listing of my bean :
public class TopBean {
     private UserBean userBean = null;
//some code other code
public void logoff() {
          HttpSession session = (HttpSession)FacesContext.getCurrentInstance().getExternalContext().getSession(false);
          if (session==null) {
               System.out.println("Session is null");
          } else {
               UserController userCon = new UserController();
               if (userCon.logoffUser(userBean)) {
                    System.out.println("successful logout");               
               } else {               
                    System.out.println("Unsuccessful logout");
I have a logoff button on a jsp page:
<h:commandButton image="images/logoff.gif" alt="Logoff" action="#{TopBean.logoff}" styleClass="linkButton" onclick="javascript:logoffConfirm()"/></td>
The session timeout is 30minutes
<session-config>
     <session-timeout>30</session-timeout>
</session-config>
If after the session times out, i decide to click on logoff so that my logged_on status in my database is updated it throws an exception that it cannot instantiate the class TopBean (i have a bit field in my user table logged_on that needs to get updated otherwise on next logged my app throws error with "user is already logged on"). My bean TopBean gets initialised directly after I log in to my app. As you can see i'm already trying to validate if the session is null, however before i can even call the method it throws "cannot instantiate class TopBean" exception. This indicates to me that once the session has expired, i clears what ever objects that were being held in that session